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QtXml Module

The QtXml module provides implementations of SAX and DOM. More...

Types

• class QDomAttr
• class QDomCDATASection
• class QDomCharacterData
• class QDomComment
• class QDomDocument
• class QDomDocumentFragment
• class QDomDocumentType
• class QDomElement
• class QDomEntity
• class QDomEntityReference
• class QDomImplementation
• class QDomNamedNodeMap
• class QDomNode
• class QDomNodeList
• class QDomNotation
• class QDomProcessingInstruction
• class QDomText
• class QXmlAttributes
• class QXmlContentHandler
• class QXmlDeclHandler
• class QXmlDefaultHandler
• class QXmlDTDHandler
• class QXmlEntityResolver
• class QXmlErrorHandler
• class QXmlInputSource
• class QXmlLexicalHandler
• class QXmlLocator
• class QXmlNamespaceSupport
• class QXmlParseException
• class QXmlReader
• class QXmlSimpleReader

from PyQt4 import QtXml

SAX is an event-based standard interface for XML parsers. The Qt interface follows the design of the SAX2 Java implementation. Its naming scheme was adapted to fit the Qt naming conventions. Details on SAX2 can be found at http://www.saxproject.org.

Support for SAX2 filters and the reader factory are under development. The Qt implementation does not include the SAX1 compatibility classes present in the Java interface. For an introduction to Qt's SAX2 classes see The Qt SAX2 classes.

DOM Level 2 is a W3C Recommendation for XML interfaces that maps the constituents of an XML document to a tree structure. The specification of DOM Level 2 can be found at http://www.w3.org/DOM/. More information about the DOM classes in Qt is provided in the Qt DOM classes.

In addition to SAX and DOM support, Qt's Q3Picture class can read and write SVG. Additional XML support is provided by the Qt Solutions group who provide, for example, classes that support SOAP and MML with the Qt XML classes.

Topics:

• The Qt SAX2 Classes
• Introduction to SAX2
• SAX2 Features
• Namespace Support via Features
• Summary
• Properties
• The Qt DOM Classes
• Introduction to DOM
• An Introduction to Namespaces
• Conventions Used in the Qt XML Documentation

    <quote>A quotation.</quote>

Whilst this is a fast and simple approach to read XML documents, manipulation is difficult because data is not stored, simply handled and discarded serially. The DOM interface reads in and stores the whole document in a tree structure; this takes more memory, but makes it easier to manipulate the document's structure..

The Qt XML module provides an abstract class, QXmlReader, that defines the interface for potential SAX2 readers. Qt includes a reader implementation, QXmlSimpleReader, that is easy to adapt through subclassing.

The reader reports parsing events through special handler classes:

These classes are abstract classes describing the interface. The QXmlDefaultHandler class provides a "do nothing" default implementation for all of them. Therefore users only need to overload the QXmlDefaultHandler functions they are interested in.

To read input XML data a special class QXmlInputSource is used.

Apart from those already mentioned, the following SAX2 support classes provide additional useful functionality:

The Qt SAX2 implementation can report whether the reader has particular functionality using the QXmlReader.hasFeature() function. Available features can be tested with QXmlReader.feature(), and switched on or off using QXmlReader.setFeature().

Consider the example

    <document xmlns:book = 'http://trolltech.com/fnord/book/'
              xmlns      = 'http://trolltech.com/fnord/' >

A reader that does not support the http://xml.org/sax/features/namespace-prefixes feature would report the element name document but not its attributes xmlns:book and xmlns with their values. A reader with the feature http://xml.org/sax/features/namespace-prefixes reports the namespace attributes if the feature is switched on.

Other features include http://xml.org/sax/features/namespace (namespace processing, implies http://xml.org/sax/features/namespace-prefixes) and http://xml.org/sax/features/validation (the ability to report validation errors).

Whilst SAX2 leaves it to the user to define and implement whatever features are required, support for http://xml.org/sax/features/namespace (and thus http://xml.org/sax/features/namespace-prefixes) is mandantory. The QXmlSimpleReader implementation of QXmlReader, supports them, and can do namespace processing.

QXmlSimpleReader is not validating, so it does not support http://xml.org/sax/features/validation.

1. Namespace prefixes and local parts of elements and attributes can be reported.
2. The qualified names of elements and attributes are reported.
3. QXmlContentHandler.startPrefixMapping() and QXmlContentHandler.endPrefixMapping() are called by the reader.
4. Attributes that declare namespaces (i.e. the attribute xmlns and attributes starting with xmlns:) are reported.

Consider the following element:

    <author xmlns:fnord = 'http://trolltech.com/fnord/'
                 title="Ms"
                 fnord:title="Goddess"
                 name="Eris Kallisti"/>

With http://xml.org/sax/features/namespace-prefixes set to true the reader will report four attributes; but with the namespace-prefixes feature set to false only three, with the xmlns:fnord attribute defining a namespace being "invisible" to the reader.

The http://xml.org/sax/features/namespaces feature is responsible for reporting local names, namespace prefixes and URIs. With http://xml.org/sax/features/namespaces set to true the parser will report title as the local name of the fnord:title attribute, fnord being the namespace prefix and http://trolltech.com/fnord/ as the namespace URI. When http://xml.org/sax/features/namespaces is false none of them are reported.

In the current implementation the Qt XML classes follow the definition that the prefix xmlns itself isn't associated with any namespace at all (see http://www.w3.org/TR/1999/REC-xml-names-19990114/#ns-using). Therefore even with http://xml.org/sax/features/namespaces and http://xml.org/sax/features/namespace-prefixes both set to true the reader won't return either a local name, a namespace prefix or a namespace URI for xmlns:fnord.

This might be changed in the future following the W3C suggestion http://www.w3.org/2000/xmlns/ to associate xmlns with the namespace http://www.w3.org/2000/xmlns.

As the SAX2 standard suggests, QXmlSimpleReader defaults to having http://xml.org/sax/features/namespaces set to true and http://xml.org/sax/features/namespace-prefixes set to false. When changing this behavior using QXmlSimpleReader.setFeature() note that the combination of both features set to false is illegal.

QXmlSimpleReader implements the following behavior:

The behavior of the entries marked with an asterisk (*) is not specified by SAX.

To set or query properties the following functions are provided: QXmlReader.setProperty(), QXmlReader.property() and QXmlReader.hasProperty().

All DOM nodes in the document tree are subclasses of QDomNode. The document itself is represented as a QDomDocument object.

Here are the available node classes and their potential child classes:

• QDomDocument: Possible children are
  • QDomElement (at most one)
  • QDomProcessingInstruction
  • QDomComment
  • QDomDocumentType
• QDomDocumentFragment: Possible children are
  • QDomElement
  • QDomProcessingInstruction
  • QDomComment
  • QDomText
  • QDomCDATASection
  • QDomEntityReference
• QDomDocumentType: No children
• QDomEntityReference: Possible children are
  • QDomElement
  • QDomProcessingInstruction
  • QDomComment
  • QDomText
  • QDomCDATASection
  • QDomEntityReference
• QDomElement: Possible children are
  • QDomElement
  • QDomText
  • QDomComment
  • QDomProcessingInstruction
  • QDomCDATASection
  • QDomEntityReference
• QDomAttr: Possible children are
  • QDomText
  • QDomEntityReference
• QDomProcessingInstruction: No children
• QDomComment: No children
• QDomText: No children
• QDomCDATASection: No children
• QDomEntity: Possible children are
  • QDomElement
  • QDomProcessingInstruction
  • QDomComment
  • QDomText
  • QDomCDATASection
  • QDomEntityReference
• QDomNotation: No children

With QDomNodeList and QDomNamedNodeMap two collection classes are provided: QDomNodeList is a list of nodes, and QDomNamedNodeMap is used to handle unordered sets of nodes (often used for attributes).

The QDomImplementation class allows the user to query features of the DOM implementation.

To get started please refer to the QDomDocument documentation.

Parts of the Qt XML module documentation assume that you are familiar with XML namespaces. Here we present a brief introduction; skip to Qt XML documentation conventions if you already know this material.

Namespaces are a concept introduced into XML to allow a more modular design. With their help data processing software can easily resolve naming conflicts in XML documents.

Consider the following example:

    <document>
    <book>
      <title>Practical XML</title>
      <author title="Ms" name="Eris Kallisti"/>
      <chapter>
        <title>A Namespace Called fnord</title>
      </chapter>
    </book>
    </document>

Here we find three different uses of the name title. If you wish to process this document you will encounter problems because each of the titles should be displayed in a different manner -- even though they have the same name.

The solution would be to have some means of identifying the first occurrence of title as the title of a book, i.e. to use the title element of a book namespace to distinguish it from, for example, the chapter title, e.g.:

    <book:title>Practical XML</book:title>

book in this case is a prefix denoting the namespace.

Before we can apply a namespace to element or attribute names we must declare it.

Namespaces are URIs like http://trolltech.com/fnord/book/. This does not mean that data must be available at this address; the URI is simply used to provide a unique name.

We declare namespaces in the same way as attributes; strictly speaking they are attributes. To make for example http://trolltech.com/fnord/ the document's default XML namespace xmlns we write

    xmlns="http://trolltech.com/fnord/"

To distinguish the http://trolltech.com/fnord/book/ namespace from the default, we must supply it with a prefix:

    xmlns:book="http://trolltech.com/fnord/book/"

A namespace that is declared like this can be applied to element and attribute names by prepending the appropriate prefix and a ":" delimiter. We have already seen this with the book:title element.

Element names without a prefix belong to the default namespace. This rule does not apply to attributes: an attribute without a prefix does not belong to any of the declared XML namespaces at all. Attributes always belong to the "traditional" namespace of the element in which they appear. A "traditional" namespace is not an XML namespace, it simply means that all attribute names belonging to one element must be different. Later we will see how to assign an XML namespace to an attribute.

Due to the fact that attributes without prefixes are not in any XML namespace there is no collision between the attribute title (that belongs to the author element) and for example the title element within a chapter.

Let's clarify this with an example:

    <document xmlns:book = 'http://trolltech.com/fnord/book/'
              xmlns      = 'http://trolltech.com/fnord/' >
    <book>
      <book:title>Practical XML</book:title>
      <book:author xmlns:fnord = 'http://trolltech.com/fnord/'
                   title="Ms"
                   fnord:title="Goddess"
                   name="Eris Kallisti"/>
      <chapter>
        <title>A Namespace Called fnord</title>
      </chapter>
    </book>
    </document>

Within the document element we have two namespaces declared. The default namespace http://trolltech.com/fnord/ applies to the book element, the chapter element, the appropriate title element and of course to document itself.

The book:author and book:title elements belong to the namespace with the URI http://trolltech.com/fnord/book/.

The two book:author attributes title and name have no XML namespace assigned. They are only members of the "traditional" namespace of the element book:author, meaning that for example two title attributes in book:author are forbidden.

In the above example we circumvent the last rule by adding a title attribute from the http://trolltech.com/fnord/ namespace to book:author: the fnord:title comes from the namespace with the prefix fnord that is declared in the book:author element.

Clearly the fnord namespace has the same namespace URI as the default namespace. So why didn't we simply use the default namespace we'd already declared? The answer is quite complex:

• attributes without a prefix don't belong to any XML namespace at all, not even to the default namespace;
• additionally omitting the prefix would lead to a title-title clash;
• writing it as xmlns:title would declare a new namespace with the prefix title instead of applying the default xmlns namespace.

With the Qt XML classes elements and attributes can be accessed in two ways: either by refering to their qualified names consisting of the namespace prefix and the "real" name (or local name) or by the combination of local name and namespace URI.

More information on XML namespaces can be found at http://www.w3.org/TR/REC-xml-names/.